Belém (the Portuguese pronunciation sounds almost like Billiam) is now a Lisbon suburb, but started out as a port near the mouth of the Tagus river. The Mosteiro is a few kilometers away, still in Belém.
Dom Manuel I of Manueline architecture fame, also commisioned this fortification that was built in the middle of the river around 1520. The canons covered the river from a stable platform, compared to a ship. As you can see, the Tower is not in the middle of the river anymore.